Hydraulic energy storage systems

ABSTRACT

A hydraulic energy storage system for vehicles to provide higher efficiency, smaller package size, lower weight, unitary construction, durability and enhanced reliability while maintaining the capability to efficiently store and restore energy at high power levels.

BACKGROUND OF THE INVENTION

(i) Field of the Invention

This invention relates to hydraulic energy storage systems and, moreparticularly, relates to hydraulic energy storage systems used invehicles to provide higher efficiency, smaller package size, lowerweight, unitary construction, durability and enhanced reliability whilemaintaining the capability to efficiently store and restore energy athigh power levels.

(ii) Description of the Related Art

Vehicles equipped with hydraulic energy storage systems have the abilityto store kinetic energy while braking, rather than dissipate it throughthe brakes, and then restore it for subsequent acceleration. Suchvehicles are commonly called “Hydraulic Hybrid” when the vehicle primemover also contributes to the energy store, or “Stored Hydraulic EnergyPropulsion” (SHEP) when only the vehicle energy is stored. Thisapplication refers to SHEP storage, but the inventions disclosed hereinmay be equally applicable to hydraulic hybrid vehicles.

The improvements of the present invention apply to hydro-pneumaticaccumulators that are normally used to store energy in SHEP vehicles,Hydraulic Hybrids and to the associated hydraulic circuitry. In linewith industry practice, the term “fluid” as used in this applicationrefers to hydraulic fluid, typically a liquid such as a speciallyformulated mineral oil The term “gas” refers to the gas used toprecharge a hydro-pneumatic accumulator, typically being dry nitrogen.

The performance and fuel economy of a vehicle, particularly one subjectto frequent stops and starts, can be improved by recovering and storingthe vehicle kinetic energy during deceleration and then restoring it,less any losses that may occur, during subsequent acceleration. SHEPsystems have a hydraulic pump/motor (P/M) that can be connected to thedrive train of the vehicle, so that the vehicle can be decelerated bypumping high pressure hydraulic fluid into a hydro-pneumatic accumulatorthereby recovering the vehicle's kinetic energy. Subsequent accelerationcan, at least in part, be achieved by using the stored kinetic energy todrive the P/M as a motor. Hydraulic hybrid systems have this samecapability with the addition of a hydraulic pump driven by the vehicleengine. This provides a more flexible system at the cost of increasedcomplexity. Importantly it provides for still further improvements infuel economy by optimising engine usage.

Hydraulic hybrid and SHEP vehicles have been the subject of many patentsand technical papers. U.S. Pat. No. 3,903,696 shows a basic SHEP system,with U.S. Pat. No. 4,760,697 being a more complex version, and U.S. Pat.No. 4,242,922 describing the basics of a hydraulic hybrid, allincorporated herein by reference.

Published technical papers covering the use of SHEP and hybrid systemsin automobiles, buses, garbage trucks, trains and other vehicles aretypified by the following papers: Mechanical power regeneration system;“Simulation of a Hydraulic Hybrid Vehicle Power Train”, ASME-Paper n73-ICT-50, Sep. 23–27, 1973; “Practical Considerations forEnergy-Storage Motor Vehicles”, published by ASME, New York, N.Y.,U.S.A. 1981; and “Studies of an Accumulator Energy-Storage AutomobileDesign with a Single Pump/Motor Unit, SAE Paper 851677 1985.

SUMMARY OF THE INVENTION

In its broad aspect, the hydraulic energy storage system of theinvention for use in a vehicle comprises a high pressure accumulator, afirst low pressure accumulator and a second low pressure accumulator inparallel, a pump/motor in fluid communication with the high pressureaccumulator and with the first and second low pressure accumulators forpumping a fluid from the first and second low pressure accumulators tothe high pressure accumulator when the pump/motor is driven in a pumpmode and for returning fluid to the first and second low pressureaccumulators when the pump/motor is in a motor mode, said pump/motorhaving a case for circulating fluid therethrough, a first check valve inseries between the pump/motor and the second low pressure accumulatorwhen the pump/motor is in the motor mode for unidirectional flow of aportion of fluid from the pump/motor to the second low pressureaccumulator, a cooler in fluid communication in series between the caseof the pump/motor, the second low pressure accumulator, and thepump/motor, and a second check valve in series between the second lowpressure accumulator and the pump/motor case for unidirectional flow offluid from the second low pressure accumulator through pump/motor caseand the cooler to the pump/motor for cooling of said portion of thefluid when the pump/motor is in the pump mode.

More particularly, an embodiment of the hydraulic energy storage systemof the invention for use in a vehicle comprises a high pressureaccumulator, a first low pressure accumulator and a second low pressureaccumulator in parallel, a pump/motor in fluid communication with thehigh pressure accumulator and with the first and second low pressureaccumulators for pumping a fluid from the first and second low pressureaccumulators to the high pressure accumulator when the pump/motor isdriven in a pump mode and for returning fluid to the first and secondlow pressure accumulators when the pump/motor is in a motor mode, saidpump/motor having a case for circulating fluid therethrough, a firstcheck valve in series between the pump/motor, the pump case and a coolerfor unidirectional flow of a portion of fluid from the pump/motorthrough the pump case and the cooler and a second check valve in serieswith the cooler and the second low pressure accumulator forunidirectional flow of said portion of fluid from the cooler to thesecond low pressure accumulator for cooling said portion of fluid whenthe pump/motor is in the motor mode, a third check valve in series withthe second low pressure accumulator and the pump/motor and a fourthcheck valve in series with the cooler and the first check valve forunidirectional flow of a portion of fluid from the second low pressureaccumulator to the pump/motor case and through the cooler to thepump/motor for cooling said portion of the fluid when the pump/motor isin the pump mode.

An embodiment of compensated accumulator for use in a hydraulic energystorage system for use in a vehicle comprises a cylindrical housinghaving a longitudinal axis and having a high pressure chamber and a lowpressure chamber concentric with the longitudinal axis, a high pressurepiston mounted transversely in the high pressure chamber for reciprocalaxial travel in the high pressure chamber and a low pressure pistonmounted transversely in the low pressure chamber for reciprocal axialtravel in the low pressure chamber, and at least three equispaced rodsconnecting the high pressure piston to the low pressure piston formaintaining the pistons perpendicular to the longitudinal axis of thecylindrical housing during reciprocal travel.

Another embodiment of compensated accumulator for use in a hydraulicenergy storage system for use in a vehicle comprises a cylindricalhousing having a longitudinal axis with a high pressure chamber and alow pressure chamber concentric with the longitudinal axis, said lowpressure chamber having a gas end remote from the high pressure chamberand a fluid end adjacent the high pressure chamber, a high pressurepiston slidably mounted for reciprocal axial travel in the high pressurechamber and a low pressure piston mounted for reciprocal axial travel inthe low pressure chamber, at least one connecting rod for connecting thehigh pressure piston and the low pressure piston together, a firstposition sensor mounted in the low pressure chamber adjacent the lowpressure end and a second position sensor mounted in the low pressurechamber adjacent the high pressure end, whereby the first and secondposition sensors control reciprocal travel of the low pressure piston inthe low pressure chamber. The compensated accumulator may additionallycomprises a pressure sensor in fluid communication with the highpressure fluid chamber whereby the second position sensor or thepressure sensor controls reciprocal travel of the high pressure and lowpressure pistons and actuates a heating system. The first positionsensor may be mounted in the end wall and, preferably, is mounted in theend wall on the longitudinal axis and comprises an ultrasonictransducer.

Another embodiment of compensated accumulator for use in a hydraulicenergy storage system for use in a vehicle comprises a cylindricalhousing having a longitudinal axis and having a high pressure chamberand low pressure chamber concentric with said longitudinal axis, eachsaid high pressure chamber and said low pressure chamber having a gasend remote from each other and a fluid end adjacent each other, a highpressure piston slidably mounted for reciprocal axial travel in the highpressure chamber and a low pressure piston slidably mounted forreciprocal axial travel in the low pressure chamber, at least oneconnecting rod for connecting the high pressure and low pressure pistonstogether in axial alignment, a valve block at one end of the cylindricalhousing, and a high pressure conduit communicating the high pressurefluid end to the valve block and a low pressure conduit communicatingthe low pressure fluid end to the valve block. The high pressure and lowpressure conduits can be external of the cylindrical housing. The highpressure and low pressure conduits can be internal of the cylindricalhousing disposed parallel to the longitudinal axis and pass through thelow pressure piston, sealing means being provided in the low pressurepiston for slidably engaging and sealing the high pressure and lowpressure conduits.

A further embodiment of compensated accumulator for use in hydraulicenergy storage system for use in a vehicle comprises a cylindricalhousing having a longitudinal axis and having a high pressure chamberand a low pressure chamber concentric with the longitudinal axis, one ofsaid high pressure chamber and said low pressure chamber having a largerdiameter than the other, a high pressure piston slidably mounted forreciprocal travel in the high pressure chamber and a low pressure pistonslidably mounted for reciprocal travel in the low pressure cylinder, oneof said high pressure piston and low pressure piston having a largerdiameter than the other for creating a flow imbalance between the highpressure cylinder and the low pressure cylinder, a pump/motor in fluidcommunication with the high pressure chamber and with the low pressurechamber for pumping a fluid from the low pressure chamber to the highpressure chamber when the pump/motor is driven in a pump mode and forreturning fluid to the low pressure chamber when the pump/motor is in amotor mode, said pump/motor having a case for circulating fluidtherethrough, a low pressure accumulator in parallel with the lowpressure chamber for receiving and discharging a portion of fluid fromthe high pressure or low pressure chambers due to the flow imbalancebetween the high pressure cylinder and the low pressure cylinder, duringthe pump mode or the motor mode, a cooler in fluid communication withthe pump/motor casing, a first check valve in series between thepump/motor, the pump case and the cooler for unidirectional flow of aportion of fluid from the pump/motor through the pump case and thecooler and a second check valve in series with the cooler and the lowpressure accumulator for unidirectional flow of said portion of fluidfrom the cooler to the low pressure accumulator for cooling said portionof fluid when the pump/motor is in the motor mode, a third check valvein series with the low pressure accumulator and the pump/motor and afourth check valve in series with the cooler and the first check valvefor unidirectional flow of a portion of fluid from the low pressureaccumulator to the pump/motor case and through the cooler to thepump/motor for cooling a portion of the fluid when the pump/motor is inthe pump mode. The high pressure piston preferably is larger than thelow pressure piston whereby outflow from the high pressure chamber isgreater than the inflow to the low pressure chamber for maintaining ahigh fluid pressure and for creating positive flow imbalance from thehigh pressure cylinder to the low pressure cylinder.

The low pressure accumulator may be an annular chamber formed concentricwithin the low pressure chamber and contain an annular accumulatorpiston, in the form of an elongated annular ring, slidably mounted forreciprocal travel in the annular accumulator chamber.

A still further embodiment of compensated accumulator for use in ahydraulic energy storage system comprises a cylindrical housing having alongitudinal axis and having a high pressure chamber and a low pressurechamber concentric with the longitudinal axis, a high pressure pistonmounted transversely in the high pressure chamber for reciprocal axialtravel in the high pressure chamber and a low pressure annular pistonmounted transversely in the low pressure chamber for reciprocal travelin the low pressure chamber, at least three equispaced rods connectingthe high pressure piston to the low pressure piston for maintaining thepistons perpendicular to the longitudinal axis of the cylindricalhousing during reciprocal travel, a low pressure accumulator cylinderformed centrally in the low pressure chamber concentric with and withinthe low pressure annular piston, sealing means formed between the lowpressure accumulator cylinder and the annular piston whereby the annularpiston is in sliding engagement with the low pressure accumulatorpiston, a pump/motor in fluid communication with the high pressurechamber and with the low pressure chamber and the low pressureaccumulator for pumping a fluid from the low pressure chamber and fromthe low pressure accumulator to the high pressure chamber when thepump/motor is in a pump mode and for returning fluid to the low pressurechamber and to the low pressure accumulator from the high pressurechamber when the pump/motor is in a motor mode, said pump/motor having acase for circulating fluid therethrough, a cooler in fluid communicationwith the pump/motor casing and the low pressure accumulator whereby thefluid flowing to and from the low pressure accumulator flows through thecooler when the pump/motor is in the pump and motor modes. Preferably,the high pressure chamber has a steel liner for reciprocal axial travelof the high pressure piston therein, said steel liner defining anannulus between the steel liner and the cylinder substantially thelength of the piston stroke, substantially the length of the highpressure chamber and fluid conduit means interconnecting said annuluswith fluid in the high pressure chamber for equalizing hydraulicpressure between the liner and the chamber.

Another embodiment of compensated accumulator having an atmosphericchamber at the distal end of the low pressure chamber in which the lowpressure piston reciprocates comprises the piston having axial plungerextending therefrom, a surge reservoir for receiving fluid draining froma piston/motor, a cylindrical gallery formed in an end wall of the lowpressure chamber for sealingly receiving the piston plunger and forreceiving fluid from the surge reservoir for draining into theatmospheric chamber, and a fluid outlet in the bottom of the atmosphericchamber in communication with a low pressure accumulator or low pressurechamber through a check valve, whereby insertion of the piston plungercloses the atmospheric chamber to the atmosphere and compression of airin the atmospheric chamber opens the check valve to pump fluid in thebottom of the atmospheric chamber to the low pressure accumulator or lowpressure chamber.

An alternative compensated accumulator having an atmospheric chamber atthe distal end of the low pressure chamber in which the low pressurepiston reciprocates comprises a surge reservoir for receiving fluiddraining from a piston/motor, an opening formed in an end wall of thelow pressure chamber for receiving fluid from the surge reservoir fordraining into the atmospheric chamber, plunger means formed in thepiston for closing said end wall opening, and a fluid outlet in thebottom of the atmospheric chamber in communication with a low pressureaccumulator or chamber through a check valve, whereby reciprocalmovement of the piston and plunger means closes the atmospheric chamberto the atmosphere and compression of air in the atmospheric chamberopens the check valve to pump fluid in the bottom of the atmosphericchamber to the low pressure accumulator or low pressure chamber.

A still further embodiment compensated accumulator having an atmosphericchamber at the distal end of the low pressure chamber comprises a springreturn plunger pump is mounted in proximity to the top of the lowpressure piston extending into the low pressure chamber for abutmentwith a barrier wall separating the low pressure chamber from the highpressure chamber, an inlet to the plunger pump from the low pressurechamber formed in the top of the low pressure piston, a normally-closedcheck valve in the inlet for undirectional flow from the low pressurechamber into the plunger pump and an outlet from the plunger pump to theatmospheric chamber, and a normally-closed check valve in the outlet forundirectional flow from the plunger pump to the atmospheric chamber,whereby abutment of the plunger pump against the barrier wall duringreciprocal movement of the low pressure piston pumps any air present atthe top of the low pressure chamber into the atmospheric chamber. Theplunger pump may be mounted in the barrier wall and conduit means formedin the barrier wall direct pumped air to the atmosphere.

Another embodiment of compensated accumulator having an atmosphericchamber at the distal end of the low pressure chamber, in which thecylindrical housing has a barrier wall separating the high pressurechamber from the low pressure chamber, comprises a poppet valve sealedin a valve seat formed in the barrier wall and biased fornormally-closed flow from the high pressure chamber to the low pressurechamber, said poppet valve having a stem projecting into the lowpressure chamber, whereby abutment of the low pressure piston againstthe poppet stem opens the poppet valve to permit flow of high pressurefluid from the high pressure chamber into the low pressure chamber.

A still further embodiment compensated accumulator having an atmosphericchamber at the distal end of the low pressure chamber, in which thecylindrical housing has a barrier wall separating the high pressurecompensated chamber from the low pressure chamber, comprises a poppetthumb valve mounted on the high pressure piston projecting towards thebarrier wall, a valve seat for the poppet thumb valve formed on thebarrier wall in fluid communication with the low pressure chamber forreceiving the poppet thumb valve for closure before complete dischargeor high pressure fluid from the high pressure chamber, and a servosupply port formed in the barrier wall in fluid communication with thepump/motor, whereby residual high pressure fluid in the high pressurechamber after closure of the poppet thumb valve is directed to the motorpump.

A unitized accumulator system comprises the compensated accumulator hasbeen desired in which the cylindrical housing is incorporated with avalve block and with an overcentre-type pump/motor ornon-overcentre-type pump/motor for a unitary structure for directmounting to a vehicle final drive.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments of the invention will now be described with reference tothe accompanying drawings, in which:

FIG. 1 is a schematic illustration of a prior art SHEP system withatmospheric fluid reservoir,

FIG. 2 is a schematic illustration of a prior art fully sealed SHEPsystem;

FIG. 3 is a schematic illustration of the use of tube LP accumulatorswith single direction cooling flow;

FIG. 4 is a schematic illustration of the use of two LP accumulatorswith dual direction cooling flow;

FIG. 5 is a schematic illustration of a SHEP system showing alongitudinal sectional view of a compensated accumulator;

FIG. 6 is a fragmentary longitudinal sectional view of a compensatedaccumulator with multiple connecting rods;

FIG. 7 is a sectional view of the compensated accumulator shown in FIG.6 taken along line 7—7 thereof;

FIG. 8 is a fragmentary longitudinal sectional view of a compensatedaccumulator with position seasoning

FIG. 9 is a fragmentary longitudinal sectional view of a compensatedaccumulator with valve block;

FIG. 10 is a sectional view of the compensated accumulator shown in FIG.9 taken along line 10—10 thereof;

FIG. 11 is a fragmentary longitudinal sectional view of a compensatedaccumulator with internal connections;

FIG. 12 is a sectional view of the compensated accumulator shown in FIG.11 taken along the line 12—12 thereof,

FIG. 13 is a schematic illustration of a SHEP system with a longitudinalsectional view of an unbalanced compensated accumulator;

FIG. 14 is a schematic illustration of the SHEP system shown in FIG. 9with a longitudinal sectional view of an external concentric LPaccumulator and unstressed HP liner;

FIG. 15 is a longitudinal sectional view of an internal concentric LPaccumulator and unstressed HP liner;

FIG. 16 is a fragmentary longitudinal sectional view of a rechargecompression pump for sealed system;

FIG. 17 is a fragmatary sectional view of the recharge compression pumpshown in FIG. 16 in a closed position;

FIG. 18 is a schematic illustration of an atmospheric reservoir systemwith a fragmentary longitudinal sectional view of a recharge compressionpump;

FIG. 19 is a fragmentary longitudinal sectional view of a compressionpump in a LP piston;

FIG. 20 is a longitudinal sectional view of an air purged system;

FIG. 21 is a longitudinal sectional view of an end of stroke protectionapparatus;

FIG. 22 is an enlarged sectional view of the spool valve in FIG. 21 in aclosed position;

FIG. 23 is an enlarge sectional view of the spool valve shown in FIG. 21in an open position;

FIG. 24 is a fragmentary sectional view of an auxiliary high pressureservo supply;

FIG. 25 is an enlarged sectional view of the servo supply shown in FIG.24 in an open position;

FIG. 26 is an enlarged sectional view of the servo supply shown in FIG.24 in a closed position;

FIG. 27 is a side elevation, partly cut away, of a unitary constructionwith overcentre pump/motor;

FIG. 28 is a side elevation, partly cut away, of a unitary constructionwith non-overcentre pump/motor;

FIG. 29 is a side elevation, partly cut away, of a unitary constructionwith non-flexible conduits overcentre pump/motor at centre position;

FIG. 30 is a side elevation, partly cut away, of a unitary constructionwith transfer gear box; and

FIG. 31 is a side elevation, partly cut away, of an unitary constructionwith reversed pump/motor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a basic SHEP system, by way of example, consisting of a P/Munit which is connected to the drive train of a vehicle in a manner notshown, so that the P/M rotation is coupled to the vehicle motion. Energyis stored in the high pressure (HP) accumulator 2, which can be scaledoff for longer term energy storage by shut-off valve 3. This accumulatortypically has a pre-charge pressure of about 150 bar and a maximumpressure of about 350 bar but could have other pressure ratios. Pressuretransducer inputs the HP value into the control system, not shown.Because the P/M unit is typically a high speed axial piston unit, itrequires a charge pressure, typically about 10 bar, at its inlet whenpumping if cavitation is to be avoided at higher speeds. This isprovided by low pressure (LP) accumulator 5. Transducer 6 inputs the lowpressure value into the control system. Fluid entering the HPaccumulator 2 will compress the gas in chamber 2 a, thus causing thepressure to rise. At the same time fluid must leave the LP accumulator,urged by the LP gas pressure in gas chamber 5 a, so that the LPaccumulator pressure must fall. The amount of fall depends on therelative sizes of the two accumulators. Normally the LP accumulator willbe larger than the HP accumulator, so that the LP accumulator pressurerange is less than on the HP accumulator side.

As the vehicle decelerates the HP accumulator pressure will rise and theLP accumulator pressure fall, and the converse as the vehicleaccelerates. This means that normally the LP accumulator pressure willbe lowest at low vehicle speeds, and P/M rotational speeds, and highestat high speeds. Within sensible limits, this is a beneficial effect asthe P/M needs a higher inlet pressure at higher speeds to avoidcavitation when operating as a pump.

The P/M unit leaks some fluid into its case, which is drained away toreservoir 7, which is open to atmosphere through filter breather 8. Thisis required with many standard P/M designs as they are not suited tohaving any significant pressure in their case, being typically rated to1 bar. This fluid is returned to the system by charge pump 9, deliveringback to the LP accumulator side through filter 10 and cooler 11.

There are many ways that the charge pump can be operated. For example alevel switch in the reservoir can switch the pump on as the reservoirbecomes full. In this case the charge pump has a dual function ofproviding enough flow through the cooler to keep the system to anacceptable operating temperature as well as replenishing the P/M caseleakage. If the required cooling flow is greater than the leakage, thepump must be kept on and its delivery circulated back through pressurecontrol valve 12. This valve is controlled to ensure that the correctamount of fluid is stored in the LP accumulator to balance the fluidcurrently stored in the HP accumulator.

The charge pump has at all times to raise its delivery pressure fromatmospheric to LP, which presents a considerable energy waste, reducingthe overall efficiency of the storage system. In this configuration theP/M unit is capable of overcentre operation, so that it operates as apump in positive displacement, causing deceleration of the vehicle andtransferring fluid from the LP to HP accumulators. It operates as amotor in negative displacement, taking fluid from the HP accumulator.The torque of the motor is a function of the displacement value and thepressure difference, so that the driver's command is translated into adisplacement value by the control system.

Other systems use a P/M unit that is only capable of one side of centreoperation as typified in U.S. Pat. No. 4,760,697, which require someadditional control valves to change from deceleration to acceleration.

It is advantageous to fill the gas volume of the HP accumulator withelastomeric foam as this makes its operation substantially isothermal,with a considerable improvement in efficiency and less problems withhigh gas temperatures. The use of foam in the LP accumulator has littleeffect on efficiency but assists in reducing temperatures.

The HP accumulator shown in FIG. 1 is defined as a bladder type withfoam filling of the bladder in compartment 2 a. This is the acceptedart, but there are concerns about the long-term reliability of thebladder as it must deform not only itself but also the foam as theaccumulator becomes charged.

Ideally the foam is not permeable and should compress with the gas sothat there is little gas flow though the interstices of the foam.Excessive flow could damage the foam and lead to some losses ofefficiency. It is not possible to provide uniform compression with acollapsing bladder, possibly leading to deterioration of the foam andcreasing failure of the bladder. FIG. 2 shows a similar SHEP system, butusing a P/M that can take the LP in its case, up to about 10 bar. Thisis designated as a sealed SHEP system as it is sealed off fromatmospheric pressure.

The P/M 21, HP accumulator 22, shut-off valve 23, transducer 24 and LPaccumulator 25 act as described with reference to correspondingcomponents in FIG. 1. In this case the circulating pump 26 has to onlypump through the pressure drops of the filter 27 and cooler 28,resulting in a significant energy saving over the open SHEP system ofFIG. 1. Conduit 29 provides for circulation flow through case of theP/M, which is often a requirement for operations at high rotationalspeeds. However, the circulating pump is typically driven off thevehicle electrical system, which is not an efficient route for energy;from alternator to battery, to electric motor and finally to the pump.The pump could be variable speed to minimise power usage depending onsystem temperature and minimum circulation requirements, which leads tocontrol complexity and higher costs. The pump is also another potentialsource of noise.

The LP accumulator 25 is shown as a conventional piston typeaccumulator. The floating piston 30 has to be long enough to be stableand not cockle and jam in the bore. It is usual practice to hollow thepiston out on the gas side to minimise overall accumulator length, asthe gas is never fully compressed in any event. This construction is,however, not ideal for foam as the foam/gas in the piston recess willcompress, requiring flow of foam/gas from the main gallery into therecess, which will lead to distortion of the foam matrix and result ingas flow through its interstices. The HP accumulator is shown with thepiston 31 facing the other way, so that the foam/gas space compressesevenly. However there is now an unusable volume of fluid in the piston,requiring the accumulator be made longer.

The variation of LP accumulator pressure as the accumulator is chargedand discharged with HP fluid is the same as discussed with the system ofFIG. 1. In this case, however, the LP accumulator pressure also acts onthe P/M case and on its shaft seal so that, in practice, the LPaccumulator needs to be comparatively large to limit the maximum LPaccumulator pressure within the ratings of the case and shaft seal. TheP/M shaft seal is a critical component as it must remain leak free overthe range of LP accumulator pressures and P/M rotational speeds. To beacceptable in an automotive environment, it must reliably retain thesealed integrity of the system for the life of the vehicle.

FIG. 3 shows an embodiment of the present invention that removes theneed for the circulating pump in a sealed SHEP system by using two LPaccumulators; a first large accumulator 45 and a second smallaccumulator 46. The P/M 41, HP accumulator 42, shut-off valve 43 andpressure transducer 44 are as in the previous figures. As the vehicleaccelerates and the P/M is in the motor mode, fluid flows from the HPaccumulator 42 through the P/M 41 to first large LP accumulator 45 andthrough check valve 49 to the second small accumulator 46.

During deceleration when the P/M is in the pump mode, the fluid flowsfrom LP accumulators 45, 46 to the P/M to HP accumulator 42. The flowfrom the large LP accumulator 45 goes directly to the P/M, but the flowfrom the small accumulator is diverted through check valve 50, throughthe P/M case, filter 47 and cooler 48 before reaching the inlet of theP/M. This provides for circulation and cooling of a portion of the fluidduring half the cycle. The check valves 49 and 50 can be reversed toprovide circulation during acceleration if so desired. The onlyefficiency loss is the pressure loss through the filter and cooler,which can be sized to suit the actual cooling flow requirement, with thesmall accumulator is sized to provide that flow.

FIG. 4 shows a further embodiment that provides for circulation duringboth modes of operation. During acceleration when the P/M is in themotor mode, main flow of fluid is from HP accumulator 42 to LPaccumulator 45, the circulating flow passes through check valve 52, theP/M case, filter 47, cooler 48, and check valve 59 to the small LPaccumulator 46. During deceleration when the P/M is in the pump mode,main flow of fluid is from LP accumulator 45 to HP accumulator 42. Thecirculating flow passes from the small LP accumulator 46 through checkvalve 50, the P/M case, filter 47, cooler 48 and check valve 60 to theP/M inlet for transfer to HP accumulator 42. This effectively provides acirculation flow that is a small portion of the fluid flow but asubstantially fixed proportion of the main flow. The more frequent flowthrough the cooler, as compared with FIG. 3, means that the cooler canbe smaller, with a smaller flow requirement, leading to a smaller smallaccumulator 46.

A compensated or pressure compensated accumulator effectively combineshigh and low pressure into one assembly so that the flow into the HPside is off-set by the flow from the LP side. Essentially it consists oftwo piston accumulators placed together axially with the pistons joinedwith a connecting rod. U.S. Pat. No. 2,721,446 and U.S. Pat. No.3,918,498, incorporated herein by reference, describe such a device. Inits simplest form it obviates the need for the LP accumulator as flowinto the HP accumulator is fully off-set by flow from the LP piston.

FIG. 5 shows a SHEP system using a compensated accumulator 62, P/M 41,shut-off valve 43 and pressure transducer 44 as before. The compensatedaccumulator 62 consists of a cylindrical housing construction enclosinga pre-charged gas/foam filled high pressure chamber 65, with areciprocally moving piston assembly consisting of a HP piston 66, LPpiston 67 and axial connecting rod 68, all with seals as shown. Chamber69, to the left of the HP piston as viewed in FIG. 5 is connected to theSHEP HP fluid side, while chamber 70, to the right of the LP piston, isconnected to the SHEP LP fluid side. Chamber 71, to the left of the LPpiston, is connected to atmosphere through filter breather 72. Thecylindrical shape of the HP foam/gas chamber is ideal in that the gasand foam can be readily compressed together without distortion of thefoam matrix or flow of gas through the foam interstices.

Flow of HP fluid into the accumulator HP chamber 69 will cause thepiston assembly to move to the right, displacing an equal volume offluid out of the LP port 78, and drawing air in through the breather 72.Conversely, flow of HP fluid out of the accumulator 62 will cause thepiston assembly to move to the left, drawing an equal volume of LP fluidin to LP chamber 70, and pushing air out through the breather 72. Asmall LP accumulator 75 is required to ensure that a suitable chargepressure is maintained at the P/M inlet and to compensate for volumevariations due to changing system temperature and other factors. Thereis no flow in and out of this accumulator during a normal decelerationand acceleration cycle, so a circulating pump 76 is required. Incontrast to the equivalent system illustrated in FIG. 2, there is novariation of LP as the accumulator is charged and discharged, whichmeans that the LP accumulator pressure has to be at all times highenough for the P/M at its highest operating speed.

The piston area 73, on the left side of the HP piston, is less than area74, on the right side, due to the presence of the connecting rod 68. Theconnecting rod is held in slight tension by the low pressure in LPchamber 70 acting on the right side of the LP piston. The force balanceof the piston assembly means that the HP fluid pressure will always beslightly higher than the HP gas pressure, by an amount dependingprimarily on the relative size of the connecting rod. The smaller thediameter of connecting rod, the smaller the difference between the fluidand gas pressures. A fluid pressure higher than the gas pressure isbeneficial, within sensible limits, as the piston seal will always actin the same direction regardless of the direction of piston movement, itwill be better lubricated and it is easier to seal a relatively highviscosity fluid rather than a low viscosity gas.

The pistons 66 and 67 can be short in length, as compared with pistons30 and 31 (FIG. 2), because they are stabilised by being joined byconnecting rod 68, providing that the rod is both of sufficient diameterand adequately connected to provide stable support of the pistons. Inpractice this means that the connecting rod needs be larger thanrequired to simply resist the small tensile force from the LP acting onthe LP piston. The compensated accumulator provides for a reduction inoverall package size, by about 25% by volume, as the pistons can beshorter than those shown in FIG. 2 and the considerable LP gas volume isnot required.

FIGS. 6 and 7 show a construction for a compensated accumulator with thecentral connecting rod 68 of FIG. 5 replaced by three small diameterequispaced connecting rods 81. This provides for stable support of thepistons 82 and 83, so that they may remain short in length, whileachieving a small total connecting rod cross-sectional area. The smallcross-sectional area of the connecting rods reduces the difference ineffective area, so that the area 84 on the fluid side of the HP piston82 is only slightly less than the area 85 on the gas side, with theresult that the fluid pressure is only slightly higher than the gaspressure to achieve a force balance. Typically, a pressure difference ofabout 10 bar with a maximum accumulator pressure of 350 bar can beachieved. This is considered ideal to provide for stable sealperformance with minimum friction and wear.

It will be understood that more than three connecting rods can be usedif required to meet a convenience of construction. A typical SHEPinstallation will have the accumulator lying horizontal, as thisattitude is most easily accommodated within the vehicle structure. Mostaccumulators used for industrial energy storage are mounted vertically.The construction illustrated in FIG. 7 has a further advantage inmaintaining the rotational position of the pistons which permits theinstallation of attitude sensitive devices in the pistons, that wouldotherwise have to be installed in the accumulator housing.

Gas type accumulators are temperature sensitive. The pressure ofelastomeric foam effectively minimises the effects of temperature risefrom compression, but the ambient temperature of an accumulator in anautomobile can vary widely depending on the current weather conditionsand other factors such as the proximity of the exhaust system and theheat transfer from the hydraulic fluid to the gas. Taking for example anenergy storage system that has a design pressure range of 175 to 350bar, the P/M is sized to provide the required traction at fulldisplacement at 175 bar, with less displacement being used at higherpressures. Pressures less than 175 bar would not provide the designtraction, but can still provide some useful energy in parallel to theengine-driven vehicle drive system. Conventional control using a HPaccumulator pressure transducer would assume that the accumulator wasempty when the pressure fell to 175 bar, so this energy is notavailable.

If the HP gas is pre-charged to 175 bar at for example 60° C., thedesign working temperature, the pre-charge pressure will be about 135bar at 0° C., giving a useful storage capacity of only 75% of design ifonly the range from 175 to 350 bar is used. If the full displacement ofthe accumulator can be used from 135 bar to 350 bar, there is actuallymore energy available than when operating at design temperature, butwith available traction falling off to 75% as the accumulator fullydischarges.

The variation in the low cut-off of HP accumulator usage makes itdifficult to use a pressure transducer for control FIG. 8 shows acompensated accumulator incorporating position sensors for controlpurposes in either the atmospheric chamber or the LP fluid chamber. Theuse of proximity switches in a bladder type HP accumulator isimpractical, and difficult in a piston type due to the high pressures.

The LP piston 91 moves to the left as viewed in FIG. 8 as the storedenergy accelerates the vehicle until triggering sensor 92, indicatingthat the accumulator is empty. During deceleration, the piston 91 movesto the right until either sensor 93 is triggered indicating that theaccumulator is full or pressure transducer 94 signals that the maximumallowable pressure has been reached. This combination of positionsensors and pressure tansducer provides for the maximum usage of theaccumulator over a range of temperature conditions. Under very coldconditions, a LP transducer reading when the accumulator is empty, asindicated by sensor 92, can be used to bring in a gas heating system,not shown, as typified in U.S. Pat. No. 4,367,786, using engine coolantor exhaust. The position sensors 92 and 93 can be of any known type.Alternatively, a longer range position transducer 95, such as of theultrasonic type, can be used.

It is advantageous for a SHEP or hydraulic hybrid system to be packagedas a single sealed unit that can be installed in the vehicle as a fullyassembled and tested hardware component; and not installed piece bypiece as with a normal hydraulic system with connecting pipeworkinstalled subsequently, the system then having to be filled with fluid,pressure charged, bled of air and test run. FIGS. 5, 9 and 10 show acompensated accumulator 100 with conduits 101 and 102 connecting theaccumulator ports to a valve block 103 which contains all the valvesrequired for the SHEP system.

FIGS. 9 and 10 show the conduits 101,102 in the corners 103, 104respectively of the assembly so that the overall package dimension isnot increased. FIGS. 11 and 12 show a similar arrangement with theconduits, located inside the compensated accumulator passing throughpiston 120 to make a more compact and neater looking package. High andlow pressure conduits 111 and 112 are connected lo the valve block 113.The HP conduit 111 is communicated to the HP side of the accumulatorthough passage 114. A seal 115 in the LP piston 120 encircling conduit111 prevents leakage as the piston is reciprocated by connecting rod118.

The LP conduit 112 is communicated to the LP side of the accumulatorthrough passage 116. A seal 119 in the LP piston 120 encircling conduit112 prevents leakage as the piston reciprocates. The diameter of the LPchamber 117 can be made larger than the diameter of the HP chamber, asillustrated, to provide an equal piston area with the inclusion of theconduits.

FIG. 13 shows a compensated accumulator 121 in which the HP piston 122is larger than the LP piston 123. This means that as the accumulator ischarged during vehicle deceleration, the inflow to the HP side is notfully compensated by the outflow from the LP side, with the differencebeing made up by a small accumulator 124. Similarly, as the accumulatoris discharged during acceleration, the outflow from the HP is greaterthan the inflow to the LP, with the difference going to the smallaccumulator.

The four check valve group 125, described above with reference to FIG.4, operates so that all flow into and out of the small accumulatorpasses through the P/M case, the filter and the cooler, thus avoidingthe need for a circulating pump. The difference in piston diameters andthe size of the small accumulator can be selected to provide any desiredamount of circulation and a reasonable increase in P/M inlet pressure asthe main accumulator discharges to accelerate the vehicle. Thecirculation process would operate equally well if the LP piston were tobe larger than the HP piston, as only a difference in their sizes isrequired, but the change in P/M inlet pressure would then decrease asthe main accumulator discharges, which would be a less favoured option.

The system of FIG. 13 requires a small accumulator that is awkward tofit into a unitary construction. FIG. 14 shows an annular smallaccumulator piston 131 that is integral and concentric with the LP endof the compensated accumulator 132. The small accumulator piston 131 isan annular ring reciprocally mounted in annular 135 and has be ofsufficient length to be stable and not cockle in its annular cylinder.This assembly can be achieved without significantly increasing thepackage dimensions both because the LP piston 133 is smaller than the HPpiston 134 and because the HP end has lo be constructed with a thickerwall than the wall thickness of the LP end to withstand the highpressure.

FIG. 15 shows another embodiment of the invention using the multipleconnecting rods 141 of FIGS. 6 and 7 with the small accumulator 142mounted concentrically inside the LP piston 145 of the compensatedaccumulator. A small accumulator gas pre-charge, typically about 5 bar,is inserted through the charge valve 143 and passages 144. The LP piston145 is shown as an annular ring and can be short in length as it is heldstable by the equispaced connecting rods 141. The integration of the LPchamber integrated into the centre of the LP accumulator eliminates theneed for a separate LP accumulator to generate cooling flow. Flow fromthis central chamber is directed through the pump/motor case during boththe accelerating and braking modes.

The small accumulator access port 146 is intentionally positioned at thetop of the accumulator cylinder 142 to prevent air collecting. The smallaccumulator is shown with the gas connection at the centre plate of thecompensated accumulator. It can be reversed with the gas connection atthe end plate if this is convenient to a particular construction. Thesmall accumulator is shown concentric to the LP piston and cylinder, butmay be positioned off-centre if this is convenient to a particularconstruction.

U.S. Pat. No. 2,764,999 shows in FIG. 2 and U.S. Pat. No. 4,714,094shows in FIG. 3 accumulator constructions in which the HP gas acts onthe outside of the cylinder tube so that it is essentially free ofstress, both of which are incorporated herein by reference. Thisconstruction, particularly of the latter disclosure, would be thepreferred construction for SHEP applications were it not for the need touse elastomeric foam in the HP gas cylinders.

FIG. 15 illustrates a preferred construction for the HP end of thecompensated accumulator suitable for the use of foam. A honed steelliner 147 provides the bore for the reciprocation of the sealed HPpiston 148. In a conventional design this is part of the pressure vessel151 construction and will expand under pressure providing an increasedextrusion gap for the seal, and will be subject to any distortions thatmay either occur during manufacture or subsequently due to mountings orother external forces. In this embodiment, HP fluid, rather than gas astaught in the references, is connected to the outside of the linerthrough connecting galleries, such as designated by numeral 149, so thatthe hydraulic pressure acts equally on the outside of the liner, thusrendering it essentially free of stress Obviously this can also beapplied to single piston accumulators. The construction illustratedshows the liner having the length of the HP piston stroke with seal 150separating the fluid from the gas. Depending on the conveniences ofconstruction, the liner can extend the fill length of the HP chamber.The seal can be replaced by an adhesive bond between the liner andpressure vessel 151, particularly if the pressure vessel is of compositeconstruction. The provision of some circulation of the fluid between theliner and pressure vessel can be used to heat up the HP gas with thehydraulic fluid, to both improve the storage capacity of the accumulatorand provide some cooling for the hydraulic system.

Hydraulic systems are prone to external leakage of hydraulic fluid; andgreat care must be taken with the system design and installation toprovide a reliable solution. A SHEP system must be free of externalleakage for the life of the vehicle. One of the strategies used tominimise leakage possibilities is to minimise all external dynamic sealsand make sure that those that are unavoidable only seal at low pressure,preferably atmospheric pressure. Atmospheric seals can provide the samelevel of reliability as the engine and gearbox shaft sealsconventionally used for road vehicles.

In the context of a SHEP sealed system with compensated accumulator, asdescribed herein, there are two external dynamic seals, being the P/Mshaft seal and the LP piston seal. Both of these seals are exposed tothe LP at about 10 bar. The shaft seal is the most critical as it is aconsiderable challenge to make a rotating seal that will providedrip-free performance at such a pressure for years; both rotating andstationary, over a range of operating temperatures. It is a much easiertask to provide a seal that may have some weepage.

In this situation, it is common practice to have a second seal with adrain returning to an atmospheric reservoir. However a sealed SHEPsystem (as shown in FIG. 2 for example) does not have an atmosphericreservoir or a recharge pump to return seal weepage to the hydraulicsystem. FIGS. 16 and 17 illustrate a means for overcoming thisshortcoming by using the atmospheric chamber 161 of a compensatedaccumulator as an atmospheric reservoir and the reciprocating action ofthe LP piston 162 to function as a recharge pump. An atmospheric drain163 is provided on the P/M, by using a second shaft seal in a manner notshown but well known to rotary seal designers, which is connected to asurge reservoir 164, preferably integral with filter breather 165. Thesurge reservoir drains into gallery 166. Any leakage from the pressureshaft seal of the P/M, together with any leakage of the LP piston, willthen collect in the bottom of the atmospheric chamber 161.

The check valve 167 provides a connection from the atmospheric chamberinto the LP side 168, connected to the small accumulator 169. The checkvalve 167 is normally held closed by the LP pressure. By preference thecheck valve should be of soft-seated design to be itself free ofleakage. The LP piston moves to the left as the accumulator storage isdischarged. A plunger 170 is formed in the end of the LP piston thatengages in sealing fashion with gallery 166. FIG. 16 shows the pistonposition just before the gallery is closed off. Once the gallery isclosed off, the remaining volume in chamber 161 is closed off andfurther travel of the piston to the left as viewed in FIG. 16 willcompress the air with any fluid therein.

FIG. 17 shows the compensated accumulator fully discharged, with the HPpiston 171 at the left end of its travel. The final volume 172 at theatmospheric chamber is designed to give a compression ratio of about 4:1after it is initially sealed off and isolated by the plunger 170. Ifthere is no fluid present from leakage, the gas in the remaining volumewill be compressed to produce a pressure of about 6 bar, not enough toopen the check valve 167 against LP, so that no gas will be forced intothe hydraulic system. If there is fluid present from leakage, the volumeof air will be reduced, but the change in volume remains essentially thesame, so the compression pressure will increase, until it reaches avalue equal or greater than LP pressure, at which time some of the fluidin the bottom of the atmospheric chamber will be forced back into thehydraulic system. This provides an automatic recharge system. The surgereservoir functions to store any leakage that might occur during thetime that the main accumulator is fully discharged and there is noaccess to the atmospheric chamber.

The same system can also be applied to an open reservoir SHEP system (asshown in FIG. 1 for example) to recirculate the case drain flow from aP/M with an atmospheric pressure case, as illustrated with reference toFIG. 18. The P/M case drain 181 goes to the atmospheric reservoir 182.Overflow 183 passes to the atmospheric chamber 184 to be pumped backinto the LP accumulator 186 through check valve 185. Accumulator 186, incombination with orifice 187, acts both to maintain the LP pressure andto smooth the pulsating delivery, for each charge and discharge cyclefrom the compression pump system. This system has the advantage ofautomatically only pumping the case drain flow as it occurs.

FIG. 18 shows the compensated accumulator with equal piston diameters,but unequal piston diameters can be used if the case drain flow does notprovide sufficient circulation.

FIG. 19 shows another embodiment of a compression pump using acompensated accumulator with multiple connecting rods such that thepistons are held at a defined verticality, which allows the rechargecheck valve 191 to be mounted in the LP piston 192. A spring loadedplunger 193, in place of STEM 170 (FIG. 17), closes off the entry port194 to provide the same compression pump action as previously discussedwith reference to the embodiment of FIG. 17.

One of the functions of the reservoir in a system using an atmosphericreservoir is to allow the escape of any air that might be introducedinto the system during initial assembly and filling, or duringsubsequent servicing. It is inevitable that some pockets of air willremain in the hydraulic system after it has been filled. As thesepockets come under pressure during initial running of the system, thisair will becomes gradually dissolved in the fluid. Normal hydraulic oil,for example, contains about 10% by volume of dissolved air atatmospheric pressure. This amount increases proportionately withpressure, so saturation at two atmospheres would lead to 20% by volumewhen returned to atmospheric. As the oil entering the system issaturated at atmospheric pressure, the additional air dissolved underpressure will increase the amount of air above 10%. Then, when the oilcirculates back to the reservoir, the air will be released toatmosphere. This process provides a continual purging of air from thehydraulic system, and is an important but little known factor supportingthe effectiveness of hydraulic systems.

Sealed SHEP systems have no atmospheric reservoir, so the trapped airhas no means of escape. Present practice is to circulate the fluidthrough an atmospheric reservoir during testing, by means of a separatecirculation pump, for long enough to allow all the trapped air to beremoved. This process is uncertain and time consuming, particularly forindustrial accumulators lying on their side which trap a large amount ofair. Piston type accumulators offer an advantage in that the ports canbe readily positioned to minimise the amount of trapped air, asdiscussed with reference to FIG. 15.

Given the presence of a recharge pump, such as the types previouslydescribed, automatic air purging of a SHEP system can be achieved aswill now be described with reference to FIG. 20. With a compensatedaccumulator with multiple connecting rods, so that the verticality ofthe pistons is fixed, the connection 201 to the LP chamber 202 ispositioned towards the bottom of the accumulator, so that any free airwill tend to become trapped at the top of the LP chamber. As far aspossible, the remainder of the system, including particularly the P/Mcase and small accumulator, will be designed and connected to minimisethe trapping of air. The connection to the small accumulator 203 isshown at the top, by way of example.

A small spring return plunger pump 204 is mounted in the LP piston 205,so that it will be operated by contact with the barrier wall 206 as thesystem becomes fully charged, with piston movement fully to the right.The inlet to the plunger pump draws from the top of the LP chamber atpassage 207 to pick up any air that may be present, and then throughinlet check valve 208. The delivery of the pump passes through outletcheck valve 209 to the atmospheric chamber 210. The spring strength ofcheck valve 209 must be strong enough to hold back the LP pressure, sothat there is only flow when the plunger is operated.

Alternatively, the plunger pump could be mounted in the barrier wall andoperated by contact with the LP piston, then communicated to theatmospheric chamber by conduits. This would be required if the pistonverticality was not ensured. Any air that is pumped will pass outthrough the filter breather. Under normal conditions it will only behydraulic fluid that is pumped. If the dissolved air is greater than the10% saturation (for oil) at atmospheric pressure, this surplus air willbe released and pass out through the filter breather The recharge pumpwill then pump the fluid back into the system. Continued operation ofthe system, by cycling the fluid through the atmospheric chamber, willtend to slowly bring the dissolved air in the system towards theatmospheric saturation level.

Unless a mechanical stop is provided for the HP piston, there is adanger of over-stressing the connecting rod(s) if the accumulatorcontinues to be charged after the LP piston has reached the end of itsstroke. While the use of a position sensing system as previouslydescribed would allow for a control system to prevent this occurrence,there is advantage in having an automatic system that positively acts toprevent such an occurrence.

FIGS. 21, 22 and 23 show a preferred embodiment of such an end of strokeprotection system. FIG. 21 shows a compensated accumulator 211, with anHP piston 212 separating the HP gas 213 from the HP fluid 214. The LPpiston 215 separates the LP fluid 216 from the atmospheric chamber 217.The two pistons are connected by a number of connecting rods 218, whichpass through the centre plate 219 with seals 220. The accumulator ischarged through HP port 221, with return LP flow from LP port 222. Theseflows are reversed during discharge of the accumulator. The centre plateincorporates an end of stroke (EOS) valve 223.

Referring to FIG. 22, the EOS valve consists of a stemmed poppet valve224 engaging with a valve seat 225 formed in the centre plate. Thepoppet valve is loosely supported in valve guide 226. The valve guidehas a number of holes 227 to allow fluid flow and is retained in thecentre plate by retaining ring 228. The poppet valve is urged to theclosed position shown by spring 229 acting between the valve guide andthe poppet valve through washer 230 and retaining ring 231.

This embodiment uses a self-aligning poppet valve design with afrusto-conical seat and a mating spherical portion on the poppet valve.In addition to being closed by the spring the valve is held closed bythe action of high pressure. The piston assembly moves to the right asthe accumulator is charged until the LP piston contacts the end of thepoppet valve stem. Further movement acts to open the valve and relievethe high pressure into the low pressure, which recirculates the fluid tothe pump inlet.

FIG. 23 illustrates this action with the LP piston 232 pushing the valveopen to permit flow path 233. This system simplifies the controls of theenergy storage system as it is then permissible to fully charge theaccumulator without concern of damage, reducing the need for accurateposition sensing or accurate pressure measurement with compensation fortemperature. Providing that adequate cooling is provided, a systemincorporating this valve can provide continued vehicle braking with theP/M after the accumulator is fully charged, as the vehicle kineticenergy is converted to heating of the fluid by the throttling action ofthe EOS valve.

It is common practice to use part of the stored HP fluid as a servosupply for the control of the P/M as this removes the need to provideanother source of servo energy. However this leads to difficulties whenthe accumulator is filly discharged, as there is then no HP servoavailable This can be overcome by the use of biasing of the P/M control,by a suitable spring or other means, so that the P/M inherently comes onstroke, but it is difficult to provide a fast control response.

For this reason it is normal practice to avoid fully discharging theaccumulator during normal use so that servo pressure is alwaysavailable, with the spring bias only used for initial start-up when theaccumulator is unavoidably fully discharged. This prevents the fullenergy capability of the accumulator being used and leads to complexityof control requiring either accurate measurement of the position of thepiston assembly or accurate measurement of pressure and temperature.

FIGS. 24, 25 and 26 illustrate a preferred embodiment of a means forautomatically preventing the complete discharge of the accumulator sothat a residual amount of HP fluid is available for servo supply. FIG.24 shows a compensated accumulator as previously described with HP andLP pistons joined by a plurality of connecting rods passing through thecentre plate. There are two HP ports, a main port 241 and a servo supplyport 242. The valve assembly 243 preferably is a thumb valve illustratedmore clearly with reference to FIGS. 25 and 26 as a valve poppet 244with a cap 245, joined together by threaded means, to be slidablymounted onto a stem 246 which is attached to the HP piston by threadedmeans. A spring 247 urges the poppet assembly away from the HP piston asfar as the head 248 on the stem allows.

A valve seat 249 is incorporated in the centre plate. This seat engageswith a corresponding mating surface on the poppet as the thumb valvecloses. This embodiment uses a self-aligning poppet valve design with afrusto-conical seat and a mating spherical portion on the poppet valve.The piston assembly moves to the left as viewed in FIGS. 25 and 26 asthe accumulator discharges. As the HP piston approaches the centreplate, the thumb valve closes against the seat in the centre plate,trapping an amount of HP fluid so that it can no longer dischargethrough the main HP port. However, the servo port is still open and HPfluid is available through this port.

FIG. 26 illustrates the configuration of the thumb valve as it closesand after some servo flow has been used The HP piston 250 has nearlycontacted the centre plate 251, with the poppet sealing against the seatThe poppet assembly has partially moved down the stem against thespring. The poppet assembly is also held closed by the action of highpressure because the main port is no longer at pressure.

Further use of servo fluid would cause the HP piston to move closer tothe centre plate until they make contact. There is then no further servofluid available. The travel of the poppet assembly on the stem slightlyexceeds the closing travel of the HP piston so that the main contact isbetween the HP piston and the centre plate and not through the thumbvalve assembly. The thumb valve simplifies the controls of the energystorage system as it then permissible to fully discharge the main HPport and still retain some energy for servo operation, reducing the needfor accurate position sensing or accurate pressure measurement withcompensation for temperature. The thumb valve provides an additionaladvantage in that the pressures on each side of the seal of the HPpiston are maintained as approximately equal during normal operation,with the seal only requiring to hold the full gas precharge pressurewhen the servo allowance is fully discharged.

A SHEP energy storage system consists of a number of componentsconnected together mechanically and by fluid conduits, which is thencharged with gas under pressure and hydraulic fluid. The charging iscritical to the successful operation of the system, with the correct gaspressures and correct amount of fluid being required. After initialcharging, the fluid will be contaminated with air, which has to bepurged from the system. It is desirable from the vehicle assembly pointof view that the energy storage system be a complete unitary assemblythat is fully charged, purged and tested prior to installation, in muchthe same way as a conventional vehicle transmission. The energy storagesystem then only requires connection of the external control devices,whether electrical or mechanical, to be a fully functioning unit.

FIG. 27 shows a compact accumulator assembly 251, as described withreference to FIG. 15, incorporating a valve block 252, as described withreference to FIG. 10. An overcentre type of P/M 253 is directly mountedto the assembly with all connections within the assembly itself. Theassembly is mounted in the vehicle with resilient mounts to minimise thetransmission of noise and vibration within the assembly. The drive shaft254 can be connected to the drive train with a universal propellershaft, either at the transmission or at an axle, depending on the layoutof the vehicle. The unitary assembly can be directly mounted to thetransmission and become part of the overall engine and transmissionassembly. If the vehicle has a chassis mounted final drive, withuniversal shafts to the wheels, the unitary assembly can be directlymounted to the final drive.

FIG. 28 shows a similar arrangement with an accumulator assembly 261 andvalve block 262 as described above, and with a non-overcentre P/M 263 aspart of the assembly. The shaft 264 can be connected to the drive trainas previously described. A cooling system 265 and filter 266 are alsoshown as part of the unitary assembly. FIG. 28 shows the PressureCompensated Accumulator directly attached to the Valve Block with Filterand Cooler directly attached to the Valve Block. It also shows thePump/Motor unit directly attached to the Valve Block. This arrangementcan be reconfigured as shown in FIG. 29 with the Pump/Motor unitdirectly mounted to the Valve Block at the centre of the UnitisedAccumulator System. This allows for the complete system to be filledwith oil, bled or air and pretested before installation into thevehicle.

FIG. 29 shows such another arrangement with the valve block 271 mountedbetween the HP end 272 of the accumulator assembly and the LP end 273,and the P/M 274 mounted to the valve block as before.

FIG. 30 shows the addition of the transfer box, which allows the systemto be connected into the power train. This construction is generallysuitable for commercial vehicles such as buses where the propeller shaftconnecting the engine and transmission to the axle can be interrupted bya transfer case 281. In this embodiment the unitary assembly includesthe transfer case. The transfer case can transfer the drive throughgears, transmission chain or drive belts. Either of the shafts 282 or283 can be connected to the transmission with the other then beingconnected to the axle.

FIG. 31 shows a variation on the configuration of FIG. 30 that is moresuitable for vehicles with limited width between the chassis rails. TheP/M 291 is mounted at the front, towards the vehicle engine, andapproximately in line with the accumulator assembly. The P/M 291 ismounted on a transfer case 292 with its shaft, not shown, pointingrearward The LP end 293 of the compensated accumulator is mounted on therearward side of the transfer case, then the centre plate 294 and HP end295. Conduits 296 connect the centre plate with the P/M. Any controlvalves required can be either mounted in the centre plate or in the portblock of the pump, or some in each. The transfer shaft 297 is connectedto the vehicle engine with a conventional drive shaft. Because the P/Mis to the front, there is enough length for a shaft with universaljoints. The other shaft 298 is connected to the rear drive axle of thevehicle using another conventional drive shaft. FIG. 31 illustrates theembodiment with an over-centre P/M. A non-overcentre design, as shown inFIG. 30, can also be used. Although the unitary construction isdescribed in conjunction with a particular design of compactaccumulator, the same principles can be applied to other accumulatorarrangements.

1. A hydraulic energy storage system for use in a vehicle comprising ahigh pressure accumulator, a first low pressure accumulator and a secondlow pressure accumulator connected in parallel, a pump/motor in fluidcommunication with the high pressure accumulator and with the first andsecond low pressure accumulators for pumping a fluid from the first andsecond low pressure accumulators to the high pressure accumulator whenthe pump/motor is driven in a pump mode and for returning fluid to thefirst and second low pressure accumulators when the pump/motor is in amotor mode, said pump/motor having a case for circulating fluid therethrough, a first check valve in series between the pump/motor and thesecond low pressure accumulator when the pump/motor is in the motor modefor unidirectional flow of a portion of fluid from the pump/motor to thesecond low pressure accumulator, a cooler in fluid communication inseries between the case of the pump/motor, the second low pressureaccumulator, and the pump/motor, and a second check valve in seriesbetween the second low pressure accumulator and the pump/motor case forunidirectional flow of fluid from the second low pressure accumulatorthrough pump/motor case and the cooler to the pump/motor for cooling ofsaid portion of the fluid when the pump/motor is in the pump mode.
 2. Aunitized accumulator system comprising the compensated accumulator asclaimed in claim 1 in which the cylindrical housing is incorporated witha valve block and with an overcentre-type pump/motor ornon-overcentre-type pump/motor for a unitary structure for directmounting to a vehicle final drive.
 3. A compensated accumulator for usein a hydraulic energy storage system for use in a vehicle comprising acylindrical housing having a longitudinal axis and having a highpressure chamber and a low pressure chamber concentric with thelongitudinal axis, a high pressure piston mounted transversely in thehigh pressure chamber for reciprocal axial travel in the high pressurechamber and a low pressure piston mounted transversely in the lowpressure chamber for reciprocal axial travel in the low pressurechamber, and at least three equispaced rods connecting the high pressurepiston to the low pressure piston for maintaining the pistonsperpendicular to the longitudinal axis of the cylindrical housing duringreciprocal travel.
 4. A compensated accumulator as claimed in claim 3,having an atmospheric chamber at the distal end of the low pressurechamber in which the low pressure piston reciprocates, said low pressurepiston having axial plunger extending therefrom, a surge reservoir forreceiving fluid draining from a piston/motor, a cylindrical galleryformed in an end wall of the low pressure chamber for sealinglyreceiving the piston plunger and for receiving fluid from the surgereservoir for draining into the atmospheric chamber, and a fluid outletin the bottom of the atmospheric chamber in communication with said lowpressure accumulator or low pressure chamber through a check valve,whereby insertion of the piston plunger closes the atmospheric chamberto the atmosphere and compression of air in the atmospheric chamberopens the check valve to pump fluid in the bottom of the atmosphericchamber to the low pressure accumulator or low pressure chamber.
 5. Acompensated accumulator as claimed in claim 3 having an atmosphericchamber at the distal end of the low pressure chamber in which the lowpressure piston reciprocates, a surge reservoir for receiving fluiddraining from a piston/motor, an opening formed in an end wall of thelow pressure chamber for receiving fluid from the surge reservoir fordraining into the atmospheric chamber, plunger means formed in thepiston for closing said end wall opening, and a fluid outlet in thebottom of the atmospheric chamber in communication with said lowpressure accumulator or chamber through a check valve, wherebyreciprocal movement of the piston and plunger means closes theatmospheric chamber to the atmosphere and compression of air in theatmospheric chamber opens the check valve to pump fluid in the bottom ofthe atmospheric chamber to the low pressure accumulator or low pressurechamber.
 6. A compensated accumulator as claimed in claim 3 having anatmospheric chamber at the distal end of the low pressure chamber inwhich a spring return plunger pump is mounted in proximity to the top ofthe low pressure piston extending into the low pressure chamber forabutment with a barrier wall separating the low pressure chamber fromthe high pressure chamber, an inlet to the plunger pump from the lowpressure chamber formed in the top of the low pressure piston, anormally-closed check valve in the inlet for undirectional flow from thelow pressure chamber into the plunger pump and an outlet from theplunger pump to the atmospheric chamber, and a normally-closed checkvalve in the outlet for undirectional flow from the plunger pump to theatmospheric chamber, whereby abutment of the plunger pump against thebarrier wall during reciprocal movement of the low pressure piston pumpsany air present at the top of the low pressure chamber into theatmospheric chamber.
 7. A compensated accumulator as claimed in claim 6in which the plunger pump is mounted in the barrier wall and conduitmeans formed in the barrier wall direct pumped air to the atmosphere. 8.A compensated accumulator as claimed in claim 3 having an atmosphericchamber at the distal end of the low pressure chamber, in which thecylindrical housing has a barrier wall separating the high pressurechamber from the low pressure chamber, a poppet valve seated in a valveseat formed in the barrier wall and biased for normally-closed flow fromthe high pressure chamber to the low pressure chamber, said poppet valvehaving a stem projecting into the low pressure chamber, whereby abutmentof the low pressure piston against the poppet stem opens the poppetvalve to permit flow of high pressure fluid from the high pressurechamber into the low pressure chamber.
 9. A compensated accumulator asclaimed in claim 3 having an atmospheric chamber at the distal end ofthe low pressure chamber, in which the cylindrical housing has a barrierwall separating the high pressure compensated chamber from the lowpressure chamber, a poppet thumb valve mounted on the high pressurepiston projecting towards the barrier wall, a valve seat for the poppetthumb valve formed on the barrier wall in fluid communication with thelow pressure chamber for receiving the poppet thumb valve for closurebefore complete discharge of high pressure fluid from the high pressurechamber, and a servo supply port formed in the barrier wall in fluidcommunication with the pump/motor, whereby residual high pressure fluidin the high pressure chamber after closure of the poppet thumb valve isdirected to the motor pump.
 10. A hydraulic energy storage system foruse in a vehicle comprising a high pressure accumulator, a first lowpressure accumulator and a second low pressure accumulator connected inparallel, a pump/motor in fluid communication with the high pressureaccumulator and with the first and second low pressure accumulators forpumping a fluid from the first and second low pressure accumulators tothe high pressure accumulator when the pump/motor is driven in a pumpmode and for returning fluid to the first and second low pressureaccumulators when the pump/motor is in a motor mode, said pump/motorhaving a case for circulating fluid therethrough, a first check valve inseries between the pump/motor, the pump case and a cooler forunidirectional flow of a portion of fluid from the pump/motor throughthe pump case and the cooler and a second check valve in series with thecooler and the second low pressure accumulator for unidirectional flowof said portion of fluid from the cooler to the second low pressureaccumulator for cooling said portion of fluid when the pump/motor is inthe motor mode, a third check valve in series with the second lowpressure accumulator and the pump/motor and a fourth check valve inseries with the cooler and the first check valve for unidirectional flowof a portion of fluid from the second low pressure accumulator to thepump/motor case and through the cooler to the pump/motor for coolingsaid portion of the fluid when the pump/motor is in the pump mode.
 11. Acompensated accumulator for use in hydraulic energy storage system foruse in a vehicle comprising a cylindrical housing having a longitudinalaxis and having a high pressure chamber and a low pressure chamberconcentric with the longitudinal axis, one of said high pressure chamberand said low pressure chamber having a larger diameter than the other, ahigh pressure piston slidably mounted for reciprocal travel in the highpressure chamber and a low pressure piston slidably mounted forreciprocal travel in the low pressure cylinder, one of said highpressure piston and low pressure piston having a larger diameter thanthe other for creating a flow imbalance between the high pressurecylinder and the low pressure cylinder, a pump/motor in fluidcommunication with the high pressure chamber and with the low pressurechamber for pumping a fluid from the low pressure chamber to the highpressure chamber when the pump/motor is driven in a pump mode and forreturning fluid to the low pressure chamber when the pump/motor is in amotor mode, said pump/motor having a case for circulating fluidtherethrough, a low pressure accumulator connected in parallel with thelow pressure chamber for receiving and discharging a portion of fluidfrom the high pressure or low pressure chambers due to the flowimbalance between the high pressure cylinder and the low pressurecylinder, during the pump mode or the motor mode, a cooler in fluidcommunication with the pump/motor casing, a first check valve in seriesbetween the pump/motor, the pump case and the cooler for unidirectionalflow of a portion of fluid from the pump/motor through the pump case andthe cooler and a second check valve in series with the cooler and thelow pressure accumulator for unidirectional flow of said portion offluid from the cooler to the low pressure accumulator for cooling saidportion of fluid when the pump/motor is in the motor mode, a third checkvalve in series with the low pressure accumulator and the pump/motor anda fourth check valve in series with the cooler and the first check valvefor unidirectional flow of a portion of fluid from the low pressureaccumulator to the pump/motor case and through the cooler to thepump/motor for cooling a portion of the fluid when the pump/motor is inthe pump mode.
 12. A compensated accumulator as claimed in claim 11 inwhich the high pressure piston is larger than the low pressure pistonwhereby outflow from the high pressure chamber is greater than theinflow to the low pressure chamber for maintaining a high fluid pressureand for creating positive flow imbalance from the high pressure cylinderto the low pressure cylinder.
 13. A compensated accumulator as claimedin claim 12 in which the low pressure accumulator is an annular chamberformed concentric within the low pressure chamber, and comprising anannular accumulator piston slidably mounted for reciprocal travel in theannular accumulator chamber.
 14. A compensated accumulator as claimed inclaim 13 in which the annular accumulator piston is an elongated annularring.
 15. A compensated accumulator as claimed in claim 14 having anatmospheric chamber at the distal end of the low pressure chamber inwhich the low pressure piston reciprocates, said low pressure pistonhaving axial plunger extending therefrom, a surge reservoir forreceiving fluid draining from a piston/motor, a cylindrical galleryformed in an end wall of the low pressure chamber for sealinglyreceiving the piston plunger and for receiving fluid from the surgereservoir for draining into the atmospheric chamber, and a fluid outletin the bottom of the atmospheric chamber in communication with a lowpressure accumulator or low pressure chamber through a check valve,whereby insertion of the piston plunger closes the atmospheric chamberto the atmosphere and compression of air in the atmospheric chamberopens the check valve to pump fluid in the bottom of the atmosphericchamber to the low pressure accumulator or low pressure chamber.
 16. Acompensated accumulator as claimed in claim 14 having an atmosphericchamber at the distal end of the low pressure chamber in which the lowpressure piston reciprocates, a surge reservoir for receiving fluiddraining from a piston/motor, an opening formed in an end wall of thelow pressure chamber for receiving fluid from the surge reservoir fordraining into the atmospheric chamber, plunger means formed in thepiston for closing said end wall opening, and a fluid outlet in thebottom of the atmospheric chamber in communication with a low pressureaccumulator or chamber through a check valve, whereby reciprocalmovement of the piston and plunger means closes the atmospheric chamberto the atmosphere and compression of air in the atmospheric chamberopens the check valve to pump fluid in the bottom of the atmosphericchamber to the low pressure accumulator or low pressure chamber.
 17. Acompensated accumulator as claimed in claim 14 having an atmosphericchamber at the distal end of the low pressure chamber in which a springreturn plunger pump is mounted in proximity to the top of the lowpressure piston extending into the low pressure chamber for abutmentwith a barrier wall separating the low pressure chamber from the highpressure chamber, an inlet to the plunger pump from the low pressurechamber formed in the top of the low pressure piston, a normally-closedcheck valve in the inlet for undirectional flow from the low pressurechamber into the plunger pump and an outlet from the plunger pump to theatmospheric chamber, and a normally-closed check valve in the outlet forundirectional flow from the plunger pump to the atmospheric chamber,whereby abutment of the plunger pump against the barrier wall duringreciprocal movement of the low pressure piston pumps any air present atthe top of the low pressure chamber into the atmospheric chamber.
 18. Acompensated accumulator as claimed in claim 14 having an atmosphericchamber at the distal end of the low pressure chamber, in which thecylindrical housing has a barrier wall separating the high pressurechamber from the low pressure chamber, a poppet valve seated in a valveseat formed in the barrier wall and biased for normally-closed flow fromthe high pressure chamber to the low pressure chamber, said poppet valvehaving a stem projecting into the low pressure chamber, whereby abutmentof the low pressure piston against the poppet stem opens the poppetvalve to permit flow of high pressure fluid from the high pressurechamber into the low pressure chamber.
 19. A compensated accumulator asclaimed in claim 14 having an atmospheric chamber at the distal end ofthe low pressure chamber, in which the cylindrical housing has barrierwall separating the high pressure compensated chamber from the lowpressure chamber, a poppet thumb valve mounted on the high pressurepiston projecting towards the barrier wall, a valve seat for the poppetthumb valve formed on the barrier wall in fluid communication with thelow pressure chamber for receiving the poppet thumb valve for closurebefore complete discharge of high pressure fluid from the high pressurechamber, and a servo supply port formed in the barrier wall in fluidcommunication with the pump/motor, whereby residual high pressure fluidin the high pressure chamber after closure of the poppet thumb valve isdirected to the motor pump.
 20. A compensated accumulator for use in ahydraulic energy storage system for use in a vehicle comprising acylindrical housing having a longitudinal axis with a high pressurechamber and a low pressure chamber concentric with the longitudinalaxis, said low pressure chamber having a gas end remote from the highpressure chamber and a fluid end adjacent the high pressure chamber, ahigh pressure piston slidably mounted for reciprocal axial travel in thehigh pressure chamber and a low pressure piston mounted for reciprocalaxial travel in the low pressure chamber, at least one connecting rodfor connecting the high pressure piston and the low pressure pistontogether, a first position sensor mounted in the low pressure chamberadjacent the low pressure end and a second position sensor mounted inthe low pressure chamber adjacent the high pressure end, whereby thefirst and second position sensors control reciprocal travel of the lowpressure piston in the low pressure chamber, and a pressure sensor influid communication with the high pressure fluid chamber whereby thesecond position sensor or the pressure sensor controls reciprocal travelof the high pressure and low pressure pistons and actuates a heatingsystem.
 21. A compensated accumulator as claimed in claim 20 in whichgas end has an end wall and in which the first position sensor ismounted in said end wall.
 22. A compensated accumulator as claimed inclaim 21 in which the first position sensor is mounted in the end wallon the longitudinal axis and comprises an ultrasonic transducer.
 23. Acompensated accumulator as claimed in claim 20 having an atmosphericchamber at the distal end of the low pressure chamber in which the lowpressure piston reciprocates, said low pressure piston having axialplunger extending therefrom, a surge reservoir for receiving fluiddraining from a piston/motor, a cylindrical gallery formed in an endwall of the low pressure chamber for sealingly receiving the pistonplunger and for receiving fluid from the surge reservoir for draininginto the atmospheric chamber, and a fluid outlet in the bottom of theatmospheric chamber in communication with said low pressure accumulatoror low pressure chamber through a check valve, whereby insertion of thepiston plunger closes the atmospheric chamber to the atmosphere andcompression of air in the atmospheric chamber opens the check valve topump fluid in the bottom of the atmospheric chamber to the low pressureaccumulator or low pressure chamber.
 24. A compensated accumulator asclaimed in claim 20 having an atmospheric chamber at the distal end ofthe low pressure chamber in which the low pressure piston reciprocates,a surge reservoir for receiving fluid draining from a piston/motor, anopening formed in an end wall of the low pressure chamber for receivingfluid from the surge reservoir for draining into the atmosphericchamber, plunger means formed in the piston for closing said end wallopening, and a fluid outlet in the bottom of the atmospheric chamber incommunication with a low pressure accumulator or chamber through a checkvalve, whereby reciprocal movement of the piston and plunger meanscloses the atmospheric chamber to the atmosphere and compression of airin the atmospheric chamber opens the check valve to pump fluid in thebottom of the atmospheric chamber to the low pressure accumulator or lowpressure chamber.
 25. A compensated accumulator as claimed in claim 20having an atmospheric chamber at the distal end of the low pressurechamber in which a spring return plunger pump is mounted in proximity tothe top of the low pressure piston extending into the low pressurechamber for abutment with a barrier wall separating the low pressurechamber from the high pressure chamber, an inlet to the plunger pumpfrom the low pressure chamber formed in the top of the low pressurepiston, a normally-closed check valve in the inlet for undirectionalflow from the low pressure chamber into the plunger pump and an outletfrom the plunger pump to the atmospheric chamber, and a normally-closedcheck valve in the outlet for undirectional flow from the plunger pumpto the atmospheric chamber, whereby abutment of the plunger pump againstthe barrier wall during reciprocal movement of the low pressure pistonpumps any air present at the top of the low pressure chamber into theatmospheric chamber.
 26. A compensated accumulator as claimed in claim20 having an atmospheric chamber at the distal end of the low pressurechamber, in which the cylindrical housing has a barrier wall separatingthe high pressure chamber from the low pressure chamber, a poppet valveseated in a valve seat formed in the barrier wall and biased fornormally-closed flow from the high pressure chamber to the low pressurechamber, said poppet valve having a stem projecting into the lowpressure chamber, whereby abutment of the low pressure piston againstthe poppet stem opens the poppet valve to permit flow of high pressurefluid from the high pressure chamber into the low pressure chamber. 27.A compensated accumulator as claimed in claim 20 having an atmosphericchamber at the distal end of the low pressure chamber, in which thecylindrical housing has a barrier wall separating the high pressurecompensated chamber from the low pressure chamber, a poppet thumb valvemounted on the high pressure piston projecting towards the barrier wall,a valve seat for the poppet thumb valve formed on the barrier wall influid communication with the low pressure chamber for receiving thepoppet thumb valve for closure before complete discharge of highpressure fluid from the high pressure chamber, and a servo supply portformed in the barrier wall in fluid communication with a pump/motor,whereby residual high pressure fluid in the high pressure chamber afterclosure of the poppet thumb valve is directed to the motor pump.
 28. Acompensated accumulator for use in a hydraulic energy storage system foruse in a vehicle comprising a cylindrical housing having a longitudinalaxis and having a high pressure chamber and low pressure chamberconcentric with said longitudinal axis, each said high pressure chamberand said low pressure chamber having a gas end remote from each otherand a fluid end adjacent each other, a high pressure piston slidablymounted for reciprocal axial travel in the high pressure chamber and alow pressure piston slidably mounted for reciprocal axial travel in thelow pressure chamber, at least one connecting rod for connecting thehigh pressure and low pressure pistons together in axial alignment, avalve block at one end of the cylindrical housing, and a high pressureconduit communicating the high pressure fluid end to the valve block anda low pressure conduit communicating the low pressure fluid end to thevalve block, and in which the high pressure and low pressure conduitsare internal of the cylindrical housing disposed parallel to thelongitudinal axis and pass through the low pressure piston, additionallycomprising sealing means formed in the low pressure piston for slidablyengaging and sealing the high pressure and low pressure conduits.
 29. Acompensated accumulator as claimed in claim 28 having an atmosphericchamber at the distal end of the low pressure chamber in which the lowpressure piston reciprocates, said low pressure piston having axialplunger extending therefrom, a surge reservoir for receiving fluiddraining from a piston/motor, a cylindrical gallery formed in an endwall of the low pressure chamber for sealingly receiving the pistonplunger and for receiving fluid from the surge reservoir for draininginto the atmospheric chamber, and a fluid outlet in the bottom of theatmospheric chamber in communication with a low pressure accumulator orlow pressure chamber through a check valve, whereby insertion of thepiston plunger closes the atmospheric chamber to the atmosphere andcompression of air in the atmospheric chamber opens the check valve topump fluid in the bottom of the atmospheric chamber to the low pressureaccumulator or low pressure chamber.
 30. A compensated accumulator asclaimed in claim 28 having an atmospheric chamber at the distal end ofthe low pressure chamber in which the low pressure piston reciprocates,a surge reservoir for receiving fluid draining from a piston/motor, anopening formed in an end wall of the low pressure chamber for receivingfluid from the surge reservoir for draining into the atmosphericchamber, plunger means formed in the piston for closing said end wallopening, and a fluid outlet in the bottom of the atmospheric chamber incommunication with a low pressure accumulator or chamber through a checkvalve, whereby reciprocal movement of the piston and plunger meanscloses the atmospheric chamber to the atmosphere and compression of airin the atmospheric chamber opens the check valve to pump fluid in thebottom of the atmospheric chamber to the low pressure accumulator or lowpressure chamber.
 31. A compensated accumulator as claimed in claim 28having an atmospheric chamber at the distal end of the low pressurechamber in which a spring return plunger pump is mounted in proximity tothe top of the low pressure piston extending into the low pressurechamber for abutment with a barrier wall separating the low pressurechamber from the high pressure chamber, an inlet to the plunger pumpfrom the low pressure chamber formed in the top of the low pressurepiston, a normally-closed check valve in the inlet for undirectionalflow from the low pressure chamber into the plunger pump and an outletfrom the plunger pump to the atmospheric chamber, and a normally-closedcheck valve in the outlet for undirectional flow from the plunger pumpto the atmospheric chamber, whereby abutment of the plunger pump againstthe barrier wall during reciprocal movement of the low pressure pistonpumps any air present at the top of the low pressure chamber into theatmospheric chamber.
 32. A compensated accumulator as claimed in claim28 a having an atmospheric chamber at the distal end of the low pressurechamber, in which the cylindrical housing has a barrier wall separatingthe high pressure compensated chamber from the low pressure chamber, apoppet thumb valve mounted on the high pressure piston projectingtowards the barrier wall, a valve seat for the poppet thumb valve formedon the barrier wall in fluid communication with the low pressure chamberfor receiving the poppet thumb valve for closure before completedischarge of high pressure fluid from the high pressure chamber, and aservo supply port formed in the barrier wall in fluid communication witha pump/motor, whereby residual high pressure fluid in the high pressurechamber after closure of the poppet thumb valve is directed to the motorpump.
 33. A compensated accumulator for use in a hydraulic energystorage system for use in a vehicle comprising a cylindrical housinghaving a longitudinal axis and having a high pressure chamber and a lowpressure chamber concentric with the longitudinal axis, a high pressurepiston mounted transversely in the high pressure chamber for reciprocalaxial travel in the high pressure chamber and a low pressure annularpiston mounted transversely in the low pressure chamber for reciprocaltravel in the low pressure chamber, at least three equispaced rodsconnecting the high pressure piston to the low pressure piston formaintaining the pistons perpendicular to the longitudinal axis of thecylindrical housing during reciprocal travel, a low pressure accumulatorcylinder formed centrally in the low pressure chamber concentric withand within the low pressure annular piston, sealing means formed betweenthe low pressure accumulator cylinder and the annular piston whereby theannular piston is in sliding engagement with the low pressureaccumulator piston, a pump/motor in fluid communication with the highpressure chamber and with the low pressure chamber and the low pressureaccumulator for pumping a fluid from the low pressure chamber and fromthe low pressure accumulator to the high pressure chamber when thepump/motor is in a pump mode and for returning fluid to the low pressurechamber and to the low pressure accumulator from the high pressurechamber when the pump/motor is in a motor mode, said pump/motor having acase for circulating fluid therethrough, a cooler in fluid communicationwith the pump/motor casing and the low pressure accumulator whereby thefluid flowing to and from the low pressure accumulator flows through thecooler when the pump/motor is in the pump and motor modes.
 34. Acompensated accumulator as claimed in claim 33 in which the low pressureaccumulator cylinder has an access port formed in an upper portionthereof for venting air to the atmosphere.
 35. A compensated accumulatoras claimed in claim 34 in which the high pressure chamber has a steelliner for reciprocal axial travel of the high pressure piston therein,said steel liner defining an annulus between the steel liner and thecylinder substantially the length of the piston stroke, and fluidconduit means interconnecting said annulus with fluid in the highpressure chamber for equalizing hydraulic pressure between the liner andthe chamber.
 36. A compensated accumulator as claimed in claim 35 inwhich the liner extends substantially the length of the high pressurechamber.